Had the first seat from when I was 19st till 16st, 6ft in height on a 20 inch frame. Rode near 8k on that seat.
2nd seat had since May 2012, now weigh 15.5st, frame is 18inches.
Daily commute is 7 miles each way, of a weekend I can ride anything from 40 miles to 80 miles.
Use a set of padded inner pants for anything over 40 miles.